1. Field of the Invention
This invention relates to non contact testing methods and apparatus for measuring structural displacements and deformation, and more particularly to such methods and apparatus for use in the field of structural test engineering.
2. Discussion of Prior Art
Known methods of measuring structural displacements without the need for physical contact between the measuring apparatus and the structure under test, include holographic and holographic related methods such as Electronic Speckle Pattern Interferometry and Shearography. These methods are sensitive to small ground vibrations and accordingly require expensive and complex mounting arrangements to produce accurate results. Known optical methods of measuring deformation of a specimen induced by static or dynamic loading use the phenomena known as Moire fringes.
Moire fringes are produced when two identical optical transmission gratings, each of which consists of alternate rulings of opaque and transparent elements of equal width, are placed face to face with their rulings inclined at a small angle and viewed against a bright background. In this situation, no light will be transmitted where the opaque elements of one grating fall on the transparent elements of the other. The appearance is of a set of dark fringes crossing the gratings, these being known as Moire fringes. A simple example of the fringe patterns produced by this phenomena can be observed by viewing light through two overlaid layers of fine net material.
If one of the gratings is kept stationary and the other is moved in a direction perpendicular to its rulings the Moire fringes move in a direction parallel to the rulings by a distance which is a magnification of the distance travelled by the moving grating. Additionally if the rulings of the transmission gratings are held parallel to each other and one grating is displaced along a line normal to the rulings, shades of grey are observed which vary cyclically as displacement increases.
If the lines of one transmission grating are projected on to a surface, and viewed through the other reference grating, the observed fringes can also be used to determine displacement of the surface normal to itself. As the surface is moved away from the light source the projected lines on the surface move in one direction in the plane of the surface and as the surface moves towards the light they are moved in the opposite direction. If the projected grating lines are then viewed through the reference grating, with their rulings parallel, the whole exposed surface area is observed as one shade of grey. As the surface moves towards or away from the light source cyclically varying shades of grey are observed. The intensity of the grey shade is a function of the magnitude of the displacement normal to the surface on to which the first grating lines are projected.
The method of producing Moire fringes in which lines of one grating are projected onto a surface which is then viewed through a reference grating is known in the art as Projection Moire Interferometry (PMI). The method has been used in numerous applications including the measurement of the amplitude of vibration of an object or panel under vibration loading. For example, in U.S. Pat. No. 4,125,025 a first optical system periodically projects the image of a grating onto the vibrating object and a second optical system having an optical axis intersecting that of the first optical system shares the image projected on, and reflected from, the object and records the shared image on photographic film. Measurement of the amplitude of vibration is obtained from analysis of the form of the Moire fringe pattern produced.
The above method relies on the projection of a series of parallel lines onto the surface of the material under test and the viewing of that surface through a suitable physical reference grating. A phase map of the surface can only be obtained by moving one of the gratings through one full pitch or 360 degrees phase shift in a number of equal steps. At each of these steps the image is captured and the fractional fringe order is obtained by looking at the intensity variations of each point throughout the 360 degree phase shift.
In dynamic tests this method cannot produce satisfactory results due to the fact that each image frame requires that the specimen remains stationary for the length of time that the diffraction grating is moved through the required steps. It can therefore be seen that the Projection Moire Fringe method of displacement measurement for out of plane deflections cannot be applied to materials under dynamic test using conventional state of the art methods and equipment.